The present invention is related to a waveform equalizer having better equalization performance even in such a case that a transfer path is strongly varied, and is also related to a mobile station wireless apparatus and a mobile base station wireless apparatus, such as a mobile telephone, a car telephone (automobile telephone), and a private digital wireless communication telephone, which use this waveform equalizer so as to remove an adverse influence caused by frequency selective fading. Further, the present invention is related to a mobile communication system arranged by these mobile station wireless apparatus and base station wireless apparatus.
FIG. 15 is a block diagram for representing an arrangement of a conventional waveform equalizer. This conventional waveform equalizer is arranged by a feed-forward filter (FF filter) 9, a feed-back filter (FB filter) 10, an adder 4, and a discriminator 5. A reception signal S1 is entered into a plurality of delay elements 2. The plural delay elements 2 are cascade-connected inside the FF filter 9. A plurality (F0 to F4) of tap arrangement control switches 8 for controlling tap arrangements of the delay elements 2 are connected to the respective delay elements 2, and are connected via a weighting device 3 to the adder 4. The output of the adder 4 is inputted to the discriminator 5, and the output of the discriminator 5 constitutes an equalization output S2. On the other hand, the equalization output S2 is entered into a plurality of delay elements 7 which are cascade-connected within the FF filter 10. A plurality (B1 to B4) of tap arrangement control switches 8 for controlling tap arrangements of the delay elements 7 are connected to the respective delay elements 7, and are connected via the weighting device 3 to the adder 4.
In this case, the FF filter 9 may contribute the equalization of the components of the preceding waves rather than the equalization of the components of main waves (namely, waves having highest levels). The preceding waves are reached to this FF filter 9 earlier than the main waves. On the other hand, the FB filter 10 may contribute the equalization of the components of the delayed waves rather than the equalization of the components of main waves (namely, waves having highest levels). The delayed waves are reached to this FB filter 10 later than the main waves.
FIG. 2 represents an example of a burst structure of a reception signal entered into the waveform equalizer. In this drawing, symbols “Ta”, “Tb”, and “Tc” show reception time instants, respectively; section Ta to Tb indicates the known reference signal; and a section Tb to Tc represents random data. FIG. 6 represents an example of reception power of incoming waves (arrival waves) corresponding to the reception burst of FIG. 2, namely shows such a condition that there is substantially no variation in a transfer path. FIG. 16 represents a predicted impulse response of a transfer path, which is predicted by employing the reference signal of the section Ta to Tb of FIG. 6. FIG. 17 is a diagram for representing ON/OFF states of the tap arrangement control switch 8.
In general, the following necessary conditions are known in this field. To equalize preceding waves within an n-symbol time period with respect to main waves, the FF filter 9 necessarily requires (n+1) pieces of taps. Also, to equalize delayed waves within an n-symbol time period, the FB filter 10 necessarily requires (n) pieces of taps.
Now, considering such a case that the equalization is carried out with respect to the incoming waves as shown in FIG. 6, in the predicted impulse response of FIG. 16, since the components of the preceding waves which are temporally advanced to the main waves are present within 1 symbol time period, the taps of the FF filter may be sufficiently selected to be (1+1=) 2 taps. Also, since all of the components of the delay waves which are temporally delayed from the main waves are present within 3-symbol time period, it may be seen that the taps of the FB filter may be sufficiently selected to be 3 taps.
As previously described, the respective taps (F0 to F4 and B1 to B4) of the tap arrangement control switch 8 are set to ON/OFF states as indicated in FIG. 17, and thus, the tap arrangement of the waveform equalizer can be optimally formed with respect to the predicted impulse response of FIG. 16.
After the respective taps are set by the tap arrangement control switch 8, the reception signal S1 is sequentially stored in the respective delay elements 2 provided on the side of the FF filter 9, and then, the weighting operation by the tap coefficient of the weighting device 3 is carried out only for such a tap output that the respective taps of the tap arrangement control switch 8 are turned ON. As a result, the weighted outputs are entered to the adder 4. Furthermore, the output of the adder 4 is entered to the discriminator 5, and then, the symbol of this input signal is judged, so that the equalization output S2 is obtained. At the same time, this equalization output S2 is sequentially stored into the respective delay elements 7 provided on the side of the FB filter 10, and then, the weighting operation by the tap coefficient of the weighting device 3 is carried out only for such a tap output that the respective taps of the tap arrangement control switch 8 are turned ON. As a result, the weighted outputs are entered to the adder 4.
While the above-described equalization operation is carried out, the respective tap coefficients of the weighting device 3 are sequentially updated in such a manner that the errors produced between the signal input to the discriminator 5 and the symbol output equal to the judgment results of the discriminator 5 can be minimized.
The conventional waveform equalizer is operated in the above-described manner. That is, while the tap arrangement is controlled by the tap arrangement control switch 8, the signal equalization operation is carried out by the optimally-set tap arrangement having the necessary number of taps. This signal equalization operation may constitute the best equalizing method in such a case that the variation of the transfer path may be substantially neglected as shown in FIG. 6.
However, the optimum tap arrangement is determined based upon the predicted impulse response of the transfer path, which is predicted by utilizing the reference signal and the like of FIG. 2. As a consequence, in such a case that the variation of the transfer path is strongly emphasized due to the fading phenomenon, for instance, there are many possibilities that such an optimum tap arrangement is no longer maintained as to the rear half data portion of the data shown in FIG. 2. In this case, the equalization performance of the equalizer is considerably deteriorated.
Also, in the case that the transfer path is strongly varied, since such a time period that a ratio of carrier wave power to noise (will be referred to as a “CNR” hereinafter) becomes small is shortened, possibility is increased under which a single reception burst contains portions where CNR becomes small. As a consequence, such possibility that the CNR of the reference signal portion of FIG. 2 becomes small would be increased, and also, when the CNR becomes small, the errors contained in the predicted value of the impulse response of the transfer path are increased. This fact may impede the determination of the optimum tap arrangement, resulting in a problem.
Also, in such a case that the levels of the impulse response of the transfer path compete with each other among the incoming waves, and therefore, any one of these incoming waves can be hardly selected as the main wave, if such an incoming wave which does not constituted an optimum wave is selected as the main wave, then the equalization is carried out by employing such a not-optimally-selected tap arrangement. As a result, there is another problem that the equalization performance would be deteriorated.